1. Field of the Invention
The present invention generally relates to the field of gas sensing devices and, more particularly, to NDIR gas analyzers.
2. Description of the Prior Art
Nondispersive infrared (NDIR) gas analyzers are used for detecting the presence and concentration of various gases. The NDIR technique has long been considered one of the best methods for gas measurement. In addition to being highly specific, the NDIR gas analyzers are also very sensitive, stable and easy to maintain.
In contrast to NDIR gas sensors, which are by definition noninteractive, interactive optical gas sensors are less reliable, are generally nonspecific, and in some cases can be poisoned into a nonfunctional state.
Interactive gas sensors are generally nonspecific because the reagent being used to determine the concentration of the desired gas may react with other gases that are present. This will naturally result in false readings. Further, if the equilibrium of the reaction between the nonspecific gas and the reagent is such that the gas and reagent remain reacted even after the partial pressure of the gas drops in the environment being monitored, the sensor will no longer function properly and is poisoned.
The response time for NDIR gas sensors is also typically shorter than that for interactive gas sensors. The reason being that the kinetics of the reaction between the sample gas and reagent controls how quickly the sensor detects a change in the concentration of the gas in the environment being monitored.
Despite the fact that interactive gas sensors are unreliable and that the NDIR gas measurement technique is one of the best, NDIR gas analyzers have not enjoyed wide spread application because of their complexity and high cost of implementation.
In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source onto the detector.
In an attempt to reduce the cost and simplify the implementation of the NDIR technique, a low-cost NDIR gas sensor technique was developed. The low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued Nov. 17, 1992, to the present applicant, and hereby incorporated by reference. This diffusion-type gas sample chamber eliminates the need for: expensive optics, mechanical choppers, and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications for the NDIR technique, which were previously considered impractical because of cost and complexity, have been opened.
The diffusion-type gas sample chamber of U.S. Pat. No. 5,163,332 uses an elongated hollow tube having an inwardly-facing specularly-reflective surface that permits the tube to function as a light-pipe for transmitting radiation from a source to a detector through the sample gas. A plurality of filtering apertures in the wall of the non-porous hollow tube permit the sample gas to enter and exit freely under ambient pressure. Particles of smoke and dust of a size greater than 0.1 micron are kept out of the chamber by use of a semi-permeable membrane that spans the apertures in the hollow tube, and condensation of the sample gas is prevented by heating the sample chamber electrically to a temperature above the dew point of the gas.
Although the low-cost NDIR gas sensor technique opened a wide variety of new applications, the gas sample chamber and the corresponding gas sensor of the low-cost NDIR technique are still too large for many potential gas sensor applications. As a result, applications in which low-cost NDIR gas sensors may be used remain limited. Furthermore, while the cost of gas sensors employing the gas sample chamber of U.S. Pat. No. 5,163,332 is less than previous NDIR gas sensors requiring expensive optics, pumps, and choppers, a further reduction in the cost of NDIR gas sensors would further increase the number of applications in which such sensors are used and the frequency of their use.
Therefore, while a need exists for a compact, inexpensive NDIR gas sensor, this need has gone unfilled. Accordingly, a goal of the present invention is to further advance the NDIR technique by providing a miniaturized, reliable, and low cost NDIR gas sensor.